The present invention relates generally to amplitude limiting circuits for frequency-modulated video signals, and more particularly to a circuit for amplitude limiting a frequency-modulated video signal reproduced from a magnetic tape. The invention prevents an inversion phenomenon, in which "black" and "white" subjects are inverted in a picture reproduced by a television receiver.
In a magnetic tape, frequency modulated, video signal, recording system, the high frequency video signal components are increased by a pre-emphasis circuit, before frequency modulation, in order to decrease relatively the noise components. In general, video signal frequency modulators use a positive modulation in which a white peak of the video signal corresponds to the upper limit of a deviation of a carrier frequency modulated by the video signal. A tip of a synchronizing signal in the video signal corresponds to the lower limit of the deviation. Accordingly, if pre-emphasis is large, when the content of the video signal changes abruptly from black to white, for example, overshoots are apt to be generated in the video signal at a portion wherein a picture changes from black to white.
When the overshoots are generated, a portion of the frequency-modulated carrier corresponding to the overshoots protrude beyond the upper limit of the predetermined deviation. High frequency signal components protruding beyond the upper limit of the deviation are not readily recorded and reproduced. Thus, the level of the reproduced signal becomes small. Accordingly, information signal components are lost when the reproduced signal passes through an amplitude limiter. The signal frequency corresponding to the overshoots is lowered to an extreme degree. At that time, a portion of the signal representing "white" (normally a high frequency) becomes a low frequency which represents "black". As a result, the so-called inversion phenomenon is produced in the reproduced picture wherein an originally white portion of the picture is inverted into "black".
In order to prevent such an inversion phenomenon, a white clipping circuit for removing the overshoots has been used heretofore in the recording system. However, if there is a large white clipping quantity, a portion of the information signal is removed, whereby a good reproducing picture characteristic cannot be obtained.
It is conceivable to increase the level of the reproduced signal with respect to the high frequency band which is higher than the upper limit of the deviation in order to prevent inversion phenomenon. However, this approach is accompanied by an increasing of the noise component level and accordingly a lowering of the signal to noise ratio. Therefore, this conceivable approach is undesirable in actual practice.